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Super lightweight foam uses sunlight to harvest water from evaporation
(Nanowerk Spotlight) Only 30% of all freshwater on the planet is not locked up in ice caps or glaciers (not for much longer, though). Of that, some 20% is in areas too remote for humans to access and of the remaining 80% about three-quarters comes at the wrong time and place - in monsoons and floods - and is not always captured for use by people. The remainder is less than 0.08 of 1% of the total water on the planet.
The demand for these scarce water resources is growing even faster than that for energy. Currently, it is estimated that almost 2 billion people face water scarcity. Depending on future rates of population growth, between 2.7 billion and 3.2 billion people may be living in either water-scarce or water-stressed conditions by 2030.
Around the world, research teams like the Smart Materials group at the Italian Institute of Technology in Genova, are addressing the rising global demand of water via the development of sustainable and straightforward technologies that make use of what nature has to offer.
"We aim at reducing the use of chemicals and elaborated and energy-intensive systems," says Despina Fragouli, a scientist leading water research in the group.
"Furthermore, we look at the fact that natural disasters are increasing in intensity and frequency, so we aspire to develop water technologies that are highly portable so that they can be rapidly deployed to ensure the survival of the disaster victims," says Siew-Leng Loo, a postdoc working with Fragouli to develop alternative water technologies to address global water security.
Loo is first author of a paper in ACS Applied Materials & Interfaces ("Solar-Driven Freshwater Generation from Seawater and Atmospheric Moisture Enabled by a Hydrophilic Photothermal Foam") where the IIT team reports a hydrophilic and self-floating photothermal foam that shows high-rate evaporation without additional components, enabling an elegant and simple approach for water harvesting through solar evaporation.
photothermal foam
(a) Photograph showing a HEPF (hydrophilically enhanced photothermal foam) standing on a dandelion, demonstrating its lightweight property. SEM images showing the hierarchical pore structure from (b) the surface and (c-e) the cross section of the HEPF. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
"We chose solar-induced evaporation as a strategy to address the water scarcity issue because the Sun has so much potential energy and if properly harvested, it can provide a sustainable source of energy to power the process," Loo tells Nanowerk. "This, in addition to the abundance of seawater and the large amount of vapor trapped in the atmosphere makes the solar-induced evaporation process using our foam to be an attractive alternative technology in meeting the global water demand in various scenarios."
The photothermal foam – a three-dimensional framework of expanded graphite enmeshed within a polymer network of polyurethane and poly(sodium acrylate) – enables an elegant and simple approach for water harvesting by utilizing energy from natural sunlight.
The team points out that the foam, which is prepared via a facile, cost-effective, and highly scalable method, can be used to harvest water not only from seawater, but also from atmospheric moisture, which is an untapped alternative resource for freshwater production. They believe that this is the first study that coherently incorporates all the desirable properties for solar evaporation, water collection, and atmospheric water harvesting into a single integrated material.
The ability to produce freshwater from a diverse range of sources is highly relevant for cases where there is a high level of uncertainty regarding the availability and quality of the source water – a typical situation in the aftermath of disaster.
The lightweight of the foam, its versatility, and simple operation suggest that it can be a potent water harvesting solution for off-grid applications such for emergency drinking water response/disaster relief and for providing water supply for remote communities with limited access to freshwater sources.
As such, this could be included in the portfolio of water solutions of interest to water policy-makers and non-governmental organizations (NGOs) for implementation in programs such as disaster preparedness and water provisions in remote villages with limited access to freshwater supply. Beyond humanitarian applications, the foam can also be developed into a commercial product catered for backpacking or hiking applications.
"The versatility of our foam, plus its high solar evaporation efficiency, suggest that it can be a promising solution to provide adequate water supply that is critical for the short-term sustenance and survival of the affected population," notes Loo. "While there exist a number of water technologies developed for such applications, it is rather untypical to find one that is so simple, that can desalinate while producing water at a good yield, and powered only by natural sunlight."
Going forward, the researchers plan to further simplify the methods for fabrication of photothermal foams to ensure the ease of translating the method for large-scale fabrication. They are also exploring other lower-cost materials are available in abundance. In particular, they are considering porous materials from natural food- and agricultural-wastes.
Another effort will be to further enhance the wettability and the photothermal properties of the foam that is critical for the further enhancement of the solar evaporation rate and moisture-capture capacity.
"We are also planning to develop multifunctional photothermal materials, that is to extend the technology to treating industrial and domestic wastewaters that contains not only inorganics such as salts and heavy metals, but also volatile organics that may escape together with the steam and end up in the distillate," says Loo.
Whereas many materials scientists around the globe are focused on improving the efficiency of solar-thermal conversion and solar evaporation efficiencies. Through the efforts of these researchers, the results have been promising.
"However, another important aspect in harvesting water through the solar evaporation process is the condensation of the generated vapor," Loo concludes. "The effect of the materials and structure of the condensation chamber on the efficiency of collecting the condensed vapor is less intensely studied. This warrants more studies to be done in this area to ensure that the water production rate is not capped by the condensation rate as significant progress is being made in terms of the solar evaporation rate."
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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